Roll of a bunch of breaking changes from the native_assets_builder and
native_assets_cli upstream. Most notably:

* https://github.com/dart-lang/native/pull/946
* https://github.com/dart-lang/native/pull/1018
* https://github.com/dart-lang/native/pull/1019

This PR also updates the template in `flutter create
--template=package_ffi` to use the rewritten API.

This PR does not change any functionality in Flutter.

For reference, the same roll in the Dart SDK:

* https://dart-review.googlesource.com/c/sdk/+/357605
* https://dart-review.googlesource.com/c/sdk/+/357623

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we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] All existing and new tests are passing.

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